Background: Bladder cancer ranks as the tenth most prevalent malignancy globally, with non-muscle invasive bladder cancer (NMIBC) accounting for approximately 75% of cases. Patients diagnosed with high-risk NMIBC (HR-NMIBC) face a grim prognosis, characterized by a 5-year disease progression rate of up to 40%. Currently, the standard treatment for HR-NMIBC involves transurethral resection of bladder tumor (TURBT) followed by adjuvant intravesical Bacillus Calmette-Guérin (BCG) therapy. However, BCG infusion therapy is associated with prolonged duration, substantial costs, and a high frequency of toxic side effects. RC48-ADC, a novel humanized anti-HER2 antibody conjugated with monomethyl auristatin E, has shown promising efficacy with a manageable safety profile in patients with locally advanced or metastatic urothelial carcinoma (mUC) in a Phase II clinical trial (NCT03507166). Therefore, investigating the therapeutic potential of RC48-ADC in HR-NMIBC patients holds significant clinical importance. Methods: We conducted a retrospective analysis on patients diagnosed with HR-NMIBC exhibiting HER2 overexpression (≥ HER2 2+) who underwent adjuvant therapy with either RC48-ADC, a HER2-targeting antibody-drug conjugate, or BCG, at West China Hospital of Sichuan University between 2019 and 2023. The primary study endpoint was the twelve-month recurrence-free survival (RFS) rate, with safety assessments as secondary endpoints. Results: A total of thirty patients diagnosed with HR-NMIBC displaying HER2 overexpression were included in the study. Following TURBT, eleven patients received adjuvant therapy with RC48-ADC, while nineteen received adjuvant therapy with BCG. The median follow-up duration for patients receiving RC48-ADC and BCG adjuvant therapy was 7.3 and 16.43 months, respectively. Among patients treated with RC48-ADC, the twelve-month RFS rate was 100%, with one out of the 11 patients experiencing relapse after 14.2 months of RC48-ADC adjuvant therapy. In the BCG-treated group, the twelve-month RFS rate was 57.6%. However, Kaplan-Meier analysis revealed no statistically significant difference between these two groups (P=0.22). In the RC48-treated group, the most commonly reported treatment-related adverse events (TRAEs) included alopecia (45.5%), arthralgia (18.2%), and nausea (18.2%). A total of 27.7% of patients experienced grade 3 TRAEs, including alopecia, rash, and hypoesthesia, with no observations of grade 4 or grade 5 TRAEs. Conclusions: RC48-ADC has demonstrated promising efficacy with a manageable safety profile as an adjuvant therapy in patients with HR-NMIBC. RC48 may be an alternative adjuvant therapy for BCG in terms of the 12-month recurrence-free survival rate.